Zodiacal Light

Zodiacal Light is the name given to a singular appearance seen after sunset or before sunrise, at all seasons of the year in low latitudes, but rarely in Great Britain except in March, April, and May in the evenings, and six months later in the mornings. It is obviously due to illuminated (partly, perhaps, self-luminous) matter surrounding the sun in a very flat lenticular form, nearly coinciding with the plane of the sun's equator, and extending to a distance from the sun greater than that of the earth, since its apex is often seen more than $90^\{\backslash circ\}$ from the sun. It seems to have been first distinctly pointed out by Cassini, and was long regarded as the sun's atmosphere. But this idea is totally irreconcilable with mechanical principles; since to assume so flat a form, in spite of the enormous attraction of the sun, and its own elasticity, an atmosphere would have to revolve with a velocity so great as to dissipate it into space. The common explanation of the phenomenon is that it consists (like the rings of Saturn) of an immense assemblage of small cosmical masses, such as are continually encountering the earth in the form of aerolites or meteorites. For the dynamical stability of such a system it is only necessary that each fragment should separately describe its elliptic orbit about the sun. The mutual perturbations of the system, on account of the enormous mass of the sun, will be exceedingly small, except in the case of actual collision; but some of the planets will have a considerable effect upon it.